A linear class AB or B power amplifier requires the use of a high side power transistor and a low side power transistor as the output stage. If standard low power techniques are applied to linearly control and drive the power transistor output stage, the resulting amplifier is subject to the following disadvantages. Low gain and additional compensation components to create a dominant pole for closed loop operation. There can be high shoot-through currents and/or high quiescent power dissipation, which can become worse if operated at high frequencies. Measures typically used to avoid shoot-through currents often produce large amounts of crossover distortion to achieve class B operation.
Among the characteristics of a good class B linear power amplifier are high open loop gain, dominant pole compensation for closed loop applications and a differential input to allow flexibility of interfacing. Further characteristics are controllable slew rate and wide bandwidth, an upper gate driver capable of floating with the source of the upper device if an N-type MOS device is used as the high side output device. It is also necessary to provide protection against shoot-through, where both devices in the linear power amplifier simultaneously conduct, resulting in large supply current transients and power dissipation.
Shoot-through can occur due to overlap in transitioning from one output device to the other in response to input drive signals or due to rapidly changing voltage on the output in combination with Miller capacitance effects on the output devices. It is also desirable to minimize crossover distortion while preventing shoot-through.
The use of a high impedance current source gate drive, circuitry to control the transition of one output device to the other, and a differential input transconductance amplifier for the input stage yields a linear class B amplifier having a number of advantages. These advantages include easy integration of required components, ease of level shifting drive signals through the use of current source gate drive sources, a current source gate drive combining with the input capacitance of a MOS power device providing high gain as well as dominant pole compensation for closed loop operation, which eliminates the need for additional compensation capacitance, and controls slew rate independent of the small signal transfer function by limiting or enhancing the maximum gate drive current under large signal conditions.
In such a linear power amplifier, separate circuits controlling the transition from one output device to the other can take the form of detection of the on/off state of each power device and inhibiting the turn on of the other device allows a simplified single differential input stage for the amplifier. This inhibition of the turn on of the non-conducting device provides shoot-through protection to prevent large supply current spikes and power dissipations during transitions from changing active output devices and under rapidly changing voltage conditions of the output stage.
The advantages reflected in the use of a current source gate drive, such as inherent high gain and dominant pole compensation, using a transconductance amplifier when driving a MOS output stage are described in the co-pending patent application, Ser. No. 07/592,148, entitled "Current Mode Gate Drive for Power MOS Transistors", commonly assigned herewith.
An aspect of the present invention includes the provision of shoot-through protection with means for producing a low impedance path from the gate of each power transistor to its source conduction electrode if the gate to source voltage at the other transistor is greater than a reference value. This additional circuitry permits the use of a desired driver circuit without modification, while preventing shoot-through whether from the driver signals or from high output voltage changes.